That's a common problem. Be glad that it's only the gas gap vibrating. It has nothing to do with the box touching the stabilizer bar. This car is well-built, with lots of reinforcement, but there is always a "weak" spot. You can go around the car and touch the panels, the spot that vibrates the most is normally near the top of the fenders, in the area of the gas cap.
I don't think there is much you can do, short of getting rid of the spring closing mechanism and using velcro to hold the cap closed...
Mobridge DA2 > JBL MS-8 > ARC XDi 600/4 > JBL Gti 408 Mids & Vifa OT19 Tweeters; PG Ti2 1600.5 > Morel HCW-10 & IDMax 15